1. Field of the Invention
The invention relates to a fuel supply apparatus for an internal combustion engine, and more particularly to a fuel supply apparatus for an internal combustion engine that suppresses the generation of fuel vapor when fuel is pressurized by a high pressure fuel pump and supplied to the internal combustion engine through an injection valve.
2. Description of Related Art
In an internal combustion engine installed in a vehicle such as an automobile, a lean burn system in which an easily ignited, rich air-fuel mixture is formed only in the vicinity of a spark plug in a spark ignition type internal combustion engine, a fuel cut technique in which a fuel supply is stopped temporarily during deceleration or the like, and an idling stop technique in which the engine is stopped when the vehicle is stationary may be employed with the aim of improving a fuel efficiency and an exhaust gas purification performance. Further, a dual injection system in which a fuel injection valve for performing a port injection (to be referred to hereafter as a port injection valve) and a fuel injection valve for performing an in-cylinder injection (to be referred to hereafter as an in-cylinder injection valve) are used together may be employed to respond to more advanced requirements including an increase in output. In this type of internal combustion engine, a fuel supply apparatus that pressurizes fuel supplied from a low pressure fuel pump to a high pressure using a high pressure fuel pump and then supplies the pressurized fuel to the in-cylinder injection valve is typically provided in order to execute an in-cylinder injection, and it is therefore necessary to suppress fuel vapor lock in which fuel vapor is generated such that the fuel cannot be pressurized by the high pressure fuel pump.
An internal combustion engine that executes a port injection is also provided with a fuel supply apparatus that supplies pressurized fuel to the port injection valve from a fuel pump, and it is likewise necessary with this type of fuel supply apparatus to suppress fuel vapor lock in which the fuel discharged by the fuel pump cannot be pressurized, leading to misfires and the like.
Hence, Japanese Patent Application Publication No. 2005-076568 (JP-A-2005-076568), for example, discloses an apparatus that calculates a correction value for correcting a target fuel pressure on the basis of a variation rate (pressure variation width/central pressure) of a fuel pressure in a fuel supply pipe for supplying fuel to a port injection valve, detects fuel vapor lock from an increase in the variation rate (pressure variation width/central pressure), and sets the correction value such that when fuel vapor lock occurs, the target fuel pressure is increased.
Further, Japanese Patent Application Publication No. 2006-200423 (JP-A-2006-200423), for example, discloses an apparatus including a sensor that detects a pressure of fuel fed to a high pressure fuel pump from a low pressure fuel pump, wherein an air mixing amount is estimated on the basis of a pressure detection value obtained by the sensor, and an air bleeding control valve that removes air from the fuel on an upstream side of the high pressure fuel pump when the air mixing amount reaches or exceeds a predetermined value.
Furthermore, Japanese Patent Application Publication No. 2001-165013 (JP-A-2001-165013), for example, discloses an apparatus in which a return control valve and a fuel temperature sensor are respectively disposed in a return passage of a high pressure regulator that regulates a discharge pressure of a high pressure fuel pump, wherein the return control valve is capable of limiting a flow rate of return fuel passing through the return passage, first and second orifices are provided in a downstream side passage for returning fuel to a fuel tank from the return control valve, and by leading an inter-orifice fuel pressure to an opening pressure varying port of a low pressure regulator that regulates the pressure (a feed fuel pressure) of fuel delivered to the high pressure fuel pump from a low pressure fuel pump, the feed fuel pressure is increased when the return control valve is opened (at a high fuel temperature).
Furthermore, Japanese Patent Application Publication No. 2010-071244 (JP-A-2010-071244), for example, discloses an apparatus that avoids a discharge defect in a high pressure fuel pump while suppressing a power consumption of a low pressure fuel pump by setting a feed fuel pressure fed to the high pressure fuel pump at an identical value to a sum of a saturated vapor pressure and a pressure loss or, taking into account variation in fuel properties and the pressure loss, a larger value than the sum.
In the fuel supply apparatuses for an internal combustion engine described above, however, the pressure (feed fuel pressure) of the fuel supplied to the fuel injection valve is controlled to the high pressure side after pressure variation has actually occurred (for example, fuel vapor lock is detected when the fuel pressure variation rate has become sufficiently large), and therefore rotation variation and air-fuel ratio variation are likely to occur due to a reduction in a fuel injection pressure.
In response to this problem, a fuel temperature sensor may be used to detect, from the fuel temperature, that a condition in which fuel vapor is likely to be generated is not yet established. In this case, however, the fuel temperature must be detected in a plurality of locations, leading to an increase in cost.
Further, when fuel vapor generation is estimated from other sensor information, a high feed fuel pressure must be set to ensure a sufficient margin relative to comparatively large variation in the estimated value. As a result, a fuel efficiency of the engine decreases, and the fuel pump deteriorates in a comparatively short period.
Furthermore, a fuel vapor generation condition (a condition in which the fuel temperature rises to a fuel vapor generation temperature) may be stored such that the fuel pressure is increased when the condition is satisfied. Likewise in this case, however, the fuel pressure is increased more than necessary, leading to similar problems to those arising when fuel vapor generation is estimated from other sensor information.
Hence, in the fuel supply apparatuses for an internal combustion engine according to the related art described above, either fuel vapor generation is suppressed by providing temperature sensors in a large number of locations, leading to a cost increase, or the fuel pressure is increased in advance on the basis of other sensor information such that the fuel pressure cannot be modified in an appropriate and timely fashion, leading to reductions in the fuel efficiency and the lifespan of the low pressure fuel pump.